1. The Field of the Invention
This invention relates to a method for making composite pressure vessels of improved delivered tensile strength. The invention further relates to the use of surface-active agents, surfactants or compounds acting as surface-active agents to improve the fiber strength translation in composite pressure vessels and to improve prepreg uniformity by reducing the variation in strength of the composite pressure vessels.
2. Technical Background
Solid propellant rocket motor cases for missile systems, spacecraft boosters and other types of large and small high performance, lightweight pressure vessels are commonly made from fiber reinforcement and various formulations of polyepoxide resins (epoxy resins) by a filament winding process. Similarly, filament winding with both polyesters and epoxy resins has made possible production of lightweight tanks, poles, piping and the like. Historically, fiberglass has been the most common reinforcement fiber. Recently other fibers such as carbon filaments, boron filaments, and high modulus organic polymer filaments, most significantly aramid filaments, have become increasingly useful in these composite structures to take advantage of their differing and sometimes unique physical properties.
The resins utilized are typically epoxy formulations based on diglycidyl ether-bisphenol A (DGEBA), reactive low molecular weight epoxy diluents, and curing agents such as aliphatic and aromatic amines and carboxylic acid anhydrides. Both flexibilized and rigid epoxy resins have been used as matrix resins for filament wound composite structures.
In providing composite articles, such as pressure vessels, either wet winding or prepreg processes have been employed. In wet winding process, the fiber is run through a resin bath containing the resin composition whereby the fiber is coated with the composition. The resulting resin-fiber combination is then wound directly into the desired structure. The structures are then cured by polymerization initiated by heat or radiation. On the other hand, if a prepreg is to be used, the fiber or "tape" is impregnated with a curable resin composition and then wound on a spool. This prepreg is stored for winding at a future time. When the prepreg is converted into a composite article, the prepreg is typically cured by polymerization initiated by heat or radiation.
One drawback encountered in the production of composite pressure vessels has been the reduction in pressure vessel tensile strength compared to the unidirectional, axial impregnated tow tensile strength. A common measure of performance in composite pressure vessels is fiber strength translation of such tow strength to delivered tensile strength of the hoop fibers of the composite pressure vessel. Improved fiber strength translation of even a few percent is significant and valuable since fiber strength translation directly effects the design, weight, strength and cost of such pressure vessels. Thus, a highly desirable object would be to increase the tow or fiber strength translation into delivered tensile strength of hoop fibers of composite pressure vessels expressed as a percent of the tow strength.
A further drawback resides in the variation of the material from which the composite pressure vessels are produced. For pressure vessels the material strength used in designing (or design allowable strength) is the average strength of the test pressure vessels less three times the standard deviation (or coefficient of variation when expressed as a percent). For example, if the average strength is 90% of the tow strength and the standard deviation is 3%, the design allowable strength is 82% of the tow strength (i.e., 90(100-3(3))/100=82%). Historical precedent suggests that composite pressure vessels fabricated by wet-winding have standard deviations of about 4 to 8% while pressure vessels fabricated from prepregs have standard deviations of approximately 2-4%. It would therefore be highly desirable to provide matrix resin formulations and methods capable of significantly reducing the standard deviations to below these values.
Such resin formulations and methods are disclosed and claimed herein.